Known methods for non-destructively inspecting an inspection object for a defect, such as a crack or the like, that occurs therein include the eddy-current flaw detection method (ECT: Eddy Current Testing) and the ultrasonic flaw detection method (UT: Ultrasonic Testing).
The eddy-current flaw detection method is a technique in which an eddy current is generated in an inspection object by generating flux changes with an exciting coil supplied with an excitation current; additionally, detection signals that represent the flux generated by this eddy current are obtained as output signals from a detection coil; and the position, shape, depth, etc. of the defect (damage) in the inspection object are determined on the basis of these detection signals.
Although the eddy-current flaw detection method performs flaw detection by detecting changes in the intensity and current pattern of the eddy current caused by the defect in the inspection object, changes in such intensity and current pattern of the eddy current are caused not only by the defect in the inspection object but also by changes in coil impedance due to fluctuations in the electrical resistivity and magnetic permeability of the inspection object, the coil orientation (distance and angle relative to the inspection object), and so on. Therefore, such changes in the coil impedance appear in the detection signals as noise, and deterioration of flaw-detection precision due to this noise has been a problem.
In the related art, for example, methods like the following have been proposed as methods for discriminating noise included in detection signals.
For example, Patent Literature 1 discloses a technique wherein, in a defect distinguishing method in which a damage signal and a noise signal are distinguished by using detection signals obtained with an eddy-current flaw-detection multiprobe in which detection coils and exciting coils are disposed side by side, X-scan signals from the exciting coils and the detection coils that are disposed in one direction of the multiprobe and Y-scan signals from the exciting coils and the detection coils that are disposed in another direction are calculated as phase angles; the calculated phase angles are plotted on a graph by setting the X-scan phase angle on the horizontal axis and the Y-scan phase angle on the vertical axis; and the noise signals included in the detection signals are distinguished based on differences in characteristics of individual detection signal extents on the graph.
Patent Literature 2 discloses noise discrimination in which a distance sensor is added to measure a distance between the sensor and a surface of the inspection object, and noise included in the detection signals is distinguished by determining whether a change occurs in the distance based on measurement results from the distance sensor.
Patent Literature 3 discloses a technique in which a normal probe 11 and a magnetic saturation probe 13 which reduces noise due to changes in magnetic permeability with a magnet provided therein are provided; signal waveforms obtained by scanning the same location with the normal probe and the magnetic saturation probe are comparatively analyzed with a processing apparatus; and a distinction is made as to whether the signal waveforms are caused by damage in the inspection object, which is a measurement target, or are caused by noise.